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A method for estimating the effects of drainage water management (DWM) or controlled drainage (CD) on nitrogen (N) losses in drainage water was evaluated using a DRAINMOD simulation analysis. The method assumes that the effect of DWM on N losses is proportional to its effect on drainage volume. The analysis was conducted for a Portsmouth sandy loam (sl) in eastern North Carolina. DRAINMOD-NII simulations predicted that DWM would reduce 35-yr average N losses to surface waters by 37% for continuous corn and 34% for a corn-wheat-soybean rotation. The effectiveness of DWM on reducing annual N losses varied from 18 to 58% over the 35 year period. Results showed that the approximate method could be used to estimate the effect of DWM on annual N loads. The method estimated the annual effect of DWM on N losses within 3 kg/ha of that predicted by DRAINMOD-NII in 18 of 35 years. Over-estimated effects in some years were balanced by under-estimated effects in others, so that the method accurately predicted the effect of DWM in the long run. However, the accuracy of the approximate method is dependent on determining the average flow weighted N concentration of the drainage water. Overall, the approximate method appears to be a promising means of assessing impacts of DWM for purposes of nutrient trading.